235 - university of the witwatersrand
TRANSCRIPT
VIBR
ATIO
N SE
VERI
TY
235
cone vortex problem is that pressure pulsation? are
unusually strong. Another identifying characteristic is
that vibration affects only ducting downstream of the fan
and the fan casing. Vestinghouse alto reports that the
pressure wave has a frequency two and half times fan
running speed.
V. VANE 0 P E N IN 6 FIGURE 8 . 3 4
The most common solution to an inlet cone vortex if, the
addition of radial dorsal fins. A report published by
L & C Steinmuller (50) on pressure pulsations in the
discharge ducts for the draught group fani. at Duvha
revealed the following. Pressure pulsations in the
diacharge ducts were some 66X lower for fans fifted with
dorsal fins than for fans without.
23C
There is little or no connection between bearing
vibration and duct vibration. At Tutuka Power Station
excessive vibration existed in the ducting from the two
ID fans. Vibration tests were performed by the nuilior on
the main vertical support channels, the cross struts and
on the fans bearings. Figures 8.35, 8.36 and 8.37 show
the frequency spectra that were obtained from one of the
main vertical support channels, a cross strut and from
the non-drive end bearing (horizontal direction),
respectively. The overall RMS on the main vertical
support channel was 2,83 mm/s, while on the cross beam it
was as high as 10,1 mm/s. On the non-drive end bearing
of the fan in the horizontal direction it was as low as
1,11 . As can be seen large vibration exists in the duct
work (10,1 mm/s RMS) with comparatively small scale
bearing vibration (1,11 mm/s RMS). There is little
structural tie between the two. If both the bearings and
the ducts vibrate, then there are two problems to be
solved.
8.6.6 Load Contours for PA Fans
Generally the horizontal overall RMS velocity values- are
higher than in the vertical direction for a particular
bearing. This can be expected because the fan is less stiff
in the horizontal direction than in the vertical direction.
If we consider only the normal operating conditions i.e. 722
to 943! MCR which corresponds approximately to the overall RMS
velocity values at 802 and 1002 MCR, then the percentage
change varies from 242 to 0,12. All the values obtained are
1 ess than 2 mm/s RMS except for the left hand PA fan,
non drive-end horizontal position, whose values range from
2 ,d 6 mm/s to 2,32 mm/s RMS.
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8.6.7 Load Contours for the ID Pans
For the ID fans tested the variation in overall RMS velocity
values obtained are more obvious than for the two PA fans. No
clear pattern emerges from there contour lines. The
horizontal overall RMS velocity values for a particular
bearing are higher than the vertical values.
If ve consider only the normal operating conditions i.e. 72X
to 94Z MCR, then the percentage change varies from 257 to
0,8Z.
All the values obtained are less than 3 mm/s RMS.
8.6.8 Load Contours for the FD Fans
The load contour lines for the FD fans following a similar
pattern es the PA fans. Generally the horizontal overall RMS
velocity values are higher than for the vertical, for each
bearing.
All the values obtained are less than 3 mm/s except for the
lefthand FD fan, drive end vertical position, whose values
range from 4,56 mm/s to 2,12 mm/s RMS.
8.6.9 load Contours for the Steam Feed Pump
The overall RMS displacements from the four permanently
installed proximity probes all show a similar trend. Between
55Z MCR and 75Z MCR the vibration levele tend to fluctuate
substantially. From 40Z MCR to 55* MCR the vibration levels
remain fairly constant and the same applies between 75Z MCR
to 85Z MCR. The vibration levels from the two accelerometers
240
mounted vertically and horizontally on the drive-erd of the
turbine fluctuate considerably.
From 401 MCR to 55X MCR the main pump ia not supplying the
boiler with water and operates on leak off. The speed of
pump during leak off only varies from 3180 RPM to 4055 RPM.
At 802 MCR and 852 MCR the pump operates at a speed of
4205 RPM jnd 4311 RPM respectively.
Taking the percentage change of the maximum and minimum
vibration levels at 752, 802 and 852 MCR for each contour,
that ia, the normal operating conditions, results in the
following:- The vibration levels from the drive end x and y
proximity probes have a percentage change of 6,62 and 10,42
respectively. The vibration levels from the non-drive end x
and y proximity probes have a percentage change of 20,3* and
18,72 respectively. The vibration levels from the
accelerometers mounted vertically and horizontally on the
drive-end of the turbine have a percentage change of 27,72
and 202 respectively.
8.6.10 Load Contours for the Electric Feed Pumps
The vibration levels from the four permanently installed
proximity probes do not fluctuate substantially. For
Electric Feed F.'mp A (F.FP A) the overall RMS displacements
from the drive end and non-drive end proximity probes vary
between 8,5 micrometers and 12,6 micrometers RMS. Fo*- F.FP B
the overall RMS displacements from the drive end proximity
probes vary between 18,4 micrometers and 19,2 micrometers
RMS, while the non-drive end displacements vary between 3,2
micrometers and 5,7 micrometers RMS. The vibration levels
from the two accelerometers mounted axially at the drive-end
and non-drive-end of main pump, fluctuate substantially.
Taking the percentage change between the maximum and minimum
vibration levels at 75X, 80X and 852 MCR for each contour
results in the following:-
Electric Feed Pump A
The vibration levels from the drive-end x
non-drive end x and y proximity probes have a
change o2 11,82, IX, 1,21 and I,2X respectively.
Electric Feed Pump B
The vibration levels from the drive-end x
non-drive-end x and y proximity probes have a
change of 2,2X, 2,It 17,61 and 49,6X respectively
and y, and
percentage
and y, and
percent age
The vibration levels from the axial drive end and non-drive
end accelerometers have a percentage change of 5,5X and 22,41
r»*pect ively.
242
When formulating guidelines necessary to perform vibration
trend analysis two approaches must be considered. The first
approach is to formulate unique guidelines for each machine.
This approach accounts for the fact that every machine
exhibits a unique vibration signature. As one can appreciate
this approach is time consuming and is only economically
feasible for machines vhich are critical to the ccr.tinuul
oper* on of plant. The second approach is to formulate
general guidelines for dii:fer»nt classes of machines. For
example, a guideline for fans, pumps, turbines and motors.
This approach it practical in the sense that only a fev
guidelines exist and not one for every machine. The latter
approach was used in this discussion.
It is normal practice when formulating guidelines for trend
plots to measure the initial vibration levels and typically
allow level changes of j factor of 2,5, that is, 8 dB, for
one equality class change. This only holds provided that the
initial vibration level is classified as normal in accordance
with reputable standards like VDI 2056. The same criteria
applied to spectrum cascade trending.
It is therefore pointless and meaningless to trend vibration
data that fluctuates more than 250X due to load variations.
In this situtation if will be impossible to distinguish
between fluctuations caubeo bv load variations or increased
vibration levels caused by an impending problem.
Judging by the results obtained from all the draught group
fans the maximum and minimum percentage change was 51X and
0,1% respectively over the 802 MCR to 100X MCR operating
8.7 GENERAL DISCUSSION
243
range. Tho average percentage change being 141. For the
electrj.c and steam feed pumps the maximum and minimum
percentage change was 49,62 and 2,12 respectively over the
operating range from 752 MCR to 852 MCR. The average
percentage change being 122. It can therefore be deduced
that sensible vibration trending can be performed for the
avovementioned machines at the specified operating
conditions. When analysing trend data from these machines
the fact that up to 502 variation in vibration levels can
occur due to load fluctuations, must be borne in uind.
In an article published by Piety and Magette (51) it is
demonstrated that variations in speed and load can introduce
changes in vibration exceeding those associated with defects.
Compensation for 9uch changes stimulated by control variables
is necessary for maintaining sensitivity for defect detection
and for reducing false alarms. To detect these anomolies
they suggest that each vibration signature be tested to
determine if its deviations from the baseline signatures are
statistically significant.
To recommend a time interval between the acquisition of
vib:at ion measurements so that an acceptable lead time before
failure can be achieved requires knowledge of the following:
a) The time period between the commencement of a defect to
the detection of this defect using vibration analysis
techniques.
2 U
b) The time period between the commencement oi a defect to
this defect resulting in catastrophic failure.
c) The time period between the '‘•tection of a defect using
vibration analysis techniques to this defect resulting in
catastrophic failure.
Tt is very difficult to quantify these lead times because:
a) They will be very dependent on the initial severity of
the defect.
b) They will also depend largely on the pertinent failure
mechanisms common to the machine.
c) In some cases defects will not necessarily be detected at
the onset by vibration analysis techniques alone. Other
parameters like seal gas pressure and temperature could
indicate the commencement of a defect well before
vibration analysis.
d) It is difficult to ascertain when a defect
when it will be detected and when this defect
in catastrophic failure.
The abovementioned lead times could be obtained from well
documented and detailed maintenance history files on various
machines. Unfortunately, because vibration condition
monitoring is relatively new in ESCOM this information is no*-
ava i1ab1e.
If the literature on this subject is consulted and from the
authors' experience at SASOL it i6 recommended that the feed
pumps and draught group fans be monitored once fortnightly.
commences,
will result
245
If the trend plots show an increase or decrease then the tine
between measurements should be decreased so that the defect
can be raore closely monitored.
Vibration maintenance history files for the abovementioned
machines must be kept in view of setting a new or* more
practical time period between vibration measurements, if
necessitated by defects which commence and result in a
failure between vibration measurements.
After discussing the pertinent failure nechanisms of the feed
pump train and the draught group fans it is possible to
recommend a system whereby minimal vibration data acquisition
can be performed for acceptable protection against the
pertinent failure mechanisms. Generally, the direction that
is monitored on a bearing, is that direction in which the
machine is the least stiff. For example, turbo-generators
are monitored continuously in the vertical direction, while
draught group fans are monitored continuously in the
horizontal direction.
The fact that the feed pumps have large casing/rotor mass
ratios and hence generate more shaft motion relative to the
bearing or hearing housing. It is for this reason that shaft
relative motion, using proximity probes, be measured. Raving
discussed the pertinent failure mechanisms of the feed pumps
most of the problems manifest themselves in Bhaft relative
movement.
The draught group fans have small casing/rotor mass ratios,
and hence most of the vibration will be transmitted from the
rotor to the bearing casing. It is therefore suggested that
shaft absolute motion for the fans be monitored.
IU 6
The following systems are recommended for:
A The Draught Group Fans
Vibration measurements should be taken once fortnightly in
the horizontal direction at the drive-end and non drive-end
bearings of the motor and fan. Vibration measurements should
also be taken in the axial direction at the drive end
bearings of the motor and fan. i.e. on either side of the
coupling. When horizontal measurements are taken on the fan,
these should be taken on the side opposite to which the
permanently installed velocity transducer is mounted. Since
the periodic measurements will be taken using a velocity
transducer with a magnetic base, this m.gnet could trip the
fan if it i» brought close to the p^rmanep*ly installed
velocity transducer.
By taking measurements in the axial direction on either side
of the coupling, misalignment can immediately be detected.
Rotor bur defects in the mcror will also be detected by
measuring in the axial direction of the motor. This is
discussed in Chapter 6.
To improve the ability to diagnose defects the following
information should also be recorded and trended each time
vibration measurements are taken.
a) Bearing temperatures
b) Stator current
The FD and ID fans have double inlet impellers with 12
staggered blades for each inlet, and the PA fans have single
inlet impellers with 10 bladps each. The running speed of
247
the FD and ID is 12,33 Hz and the PA fan's running speed is
24,83 Hz. The blade passing frequencies are (12 x 12,33) and
(24 x 12,33) ■ 148 Hz and 296 Hz for the ID and FD fans and
10 x 24,83 ■ 248 Hz for the PA tans. When investigating
I
frequency spectra from the draught group fans a minimum
frequency range of 1000 Hz is reconuended.
B The Steam Feed Pump
TJ?S HflP Pu?P
Vibration measurements should be taken once fortnightly from
the 4 proximity probes mounted 45° from the vertical at the
DE and NDE bearings. Phase measurements from the fifth
proximity probe should also be taken, so that phase and shaft
orbital analysis can be performed. Axial vibration
measurements on the main pump should also be taken using a
velocity transducer but preferably an accelerometer. The
main pump has 7 vanes per impeller and has a maximum
operating speed of 5 228 RPM (87 Hz) and hence its blade
passing rrequency is 610 Hz. When investigating frequency
spectra from the main pump a minimum frequency range of 2 000
Hz is recommended. This frequency range has to be specified
but can be changed after some experience has been gained. It
is more convenient to have excess information and then reduce
it than to have insufficient right from the outset.
The rest of the steam feed pump train
The Steam Turbine
The turbine should be monitored in all three directions at
both the DE and NDE bearing with either a velocity transducer
or an accelerometer but preferably an accelerometer.
248
The Steam Bootter Pump
The booster pump should be monitored in the horizontal
direction at the DE and NDE bearing. The booster pump has a
maximum operating speed of 1 622 RPM (27 Hi) and six vanes
per impeller. The blade passing frequency is 162 Hz.
The Steamer'» Gearbox
The steamer's gearbox should be monitored in the horizontal
and axial direction at the DE and NDE bearing. This is to
protect the gearbox from misalignment and gear problems.
C The Electric Feed pump
The main pump should be monitored int the same manner as the
steam feed pump.
The rest of the electric feed pump train.
The Voith Gearbox
The 6 journal bearings should be monitored in the horizontal
direction, and the 2 thrust bearings should be monitored in
the horizontal and axial directions.
Additional parameters that should be monitored
on the feed water pumps.
Other parameters that should be monitored ar.d trended each
time vibration measurements are recorded on the feed vater
pvmps are:
a) Suction pressure.
b) Discharge pressure.
c) Speed.
d) Load.
e) Lubrication oil temperature,
i) Bearing temperatures.
g) Sealing water temperature.
The abovementioned information is available as a hard copy as
shown in Figures 8.38 and 8.39 for the steam and electric
steam feed pump respectively.
260
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TU TW PO IETSTW rai. 577 HI.
m m m mE s v o m s •• sn f a ft i i esu
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t * : * . BFPt BEARINGS *6 7 •• J 1 c3 °C
a 3a ••;•••
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ecc m un■ U
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Con- 0/> 51EXT/P,Ps Oisch • 5 Kp3
FIGURE 8 .3 8
251
MKFPDOBr STffTTOH &EOTCf»fWfl
!985-65-29U.W.&S
\ m m BEARINGS MINGS’* ; il{28-#Cr»23*C0 2f °C .\i 29*CS 24*CB S4'aC • *13 ft K T Z 23 '5G, P/P DISCHARGE j+,/23 °C|a £ :3C|o 23 °C[ 0.3ilP»
13 i'°c 4 3a *c2 23 °C 5 21 °C j.,34- °C 8 51 °C
i?.»c .17 »C 13 «C 28 «c
18 °C LUB OIL CLR 46 °C
29°C • UORK OIL CLR
FIGURE 8.39
252
8.8 CONCLUSIONS
1) The vibration level* obtained from the draught group fane
at 80* MCR and 100Z KCR, have an average percentage
change of 14Z, a maximum of 512 and a minimum of 0,1Z.
2) The vibration level# obtained from the main feedwater
pumps, between the ranges 75Z MCR to 85Z MCR, have an
average percentage change of 12Z, a maximum of 49,6Z and
a mini mu tii of 2,1Z.
3) Sensible vibration trending can be performed for the
abov.«entioned machines and specified operating
conditions. When analysing overall vibration trend data
from these machines an average percentage change of
12-14Z and a maximum percentage change of "50Z can be
expected.
4) To deduce conclusively, for all pumps and fans, what has
already been stated above requires similar tests to be
performed on numerous fans and pumps of different
complexities and load capabilities.
5) To recommend a time interval between the acquisition of
vibration measurements so that an acceptable lead time
before failure can be achieved, requires knowledge of the
maintenance history of the machine. Although the
vibration maintenance history of the feed water pumps and
draught group fans was unknown a reasonable time interval
was recommended.
6) To recommend a procedure whereby minimal vibration data
acquisition can be performed requires knowledge of the
pertinent failure mechanisms of the machine.
253
7) Baseline signatures at various loads for the machines in
question were obtained.
8) To ensure that vibration data is acquired each time at
precisely the same location and orientation on a machine,
vibration pads or studs should be attached to the bearirg
housing. These pads or studs should have an
identification number engraved on them for recording
purposes.
MODULE 6
OTHER IMPORTANT ASPECTS OF A VIBRATION
CONDITION MONITORING PROGRAMME
These include:
a) The training requirements of the maintenance personnel.
b) Continuous versus periodic monitoring.
c) Trending of vibration data.
d) Manual and computerised logging.
e) Considerations concerning the purchasing of periodic vibeation
monitoring equipment.
f) Considerations concerning the purchasing of continuous on-line
vibration monitorin/ equipment.
g) Standardised rules for measurements on rotating equipment.
9.1 TRAINING FOR VIBRATION CONDITION BASED MAINTENANCE
9.1.1 Selection of Personnel
The selection of personnel to carry out a Vibration Condition
Monitoring (V.C.M.) system is of paramount importance. They
must be dedicated, technically competent, self-disciplined,
credible, logical, capable of writing clear and concise
reports, and above all, they must not be put off by
oppos it ion.
255
The unbelievets, as with any religion, are capable of
producing a formidable oppoaition. Comments like: "I have
run my plant for years without vibration condition based
monitoring, why do I need it now?" are regularly heard. This
attitude can be overcome by careful persistent logic and
proving the point with results: i.e. gaining credibility.
Management must not expect to reap the full benefits of a
V.C.M. programme immediately. I: is the author's belief that
the full economic benefits of such a programme will only
become evident two years after the implementation of the
programme. Hence, the maintenance staff must be given enough
time to gain experience and confidence, and to become
proficient in the use of the instrumentation.
The best areas of choice for the maintenance personnel who
will perform the V.C.M. lay in the skills band covering a
good engineerirg assistant to an engineer, preferably with
machinery experience. The simple reading of a measurement
and its interpretation according to predetermined vibration
severity charts and tables is not sufficient and often causes
evaluation errors. It is therefore necessary that the
operator knows how the machines are manufactured, the history
of the machines, the more recurring failures, how these
reveal themselves within the vibration frequency spectrum and
how they have been repaired.
256
9.1.2 Training
It is the author's view that the training of personnel is
best achieved by:
a) In-house courses in basic vibration analysis techniques
and measurements.
Some vibration analysis equipment manufacturers provide
basic courses in vibration analysis and a6 such can be a
very valuable <isset to initiating a V.C.M. programme.
Unfortunately these courses can become very *le«i
orientated and hence lose their effectiveness
training programme.
It was therefore decided that ESCOM would provide its own
in-house training. The necessary expert e for this
training programme was supplied by personnel from
Dynamics & Noise and the Central Maintenance Services
(CMS).
b) Continuous on-the-iob training.
This part should involve the use of experienced personnel
in guiding, and encouraging the operators at the power
stations. It is during the early days of launching a
system that most mistakes are made and problems
encountered. It is vitally important that the operators
be able to rely on specialised groups such as Dynamics &
Noise, and CMS for assistance in solving their problems.
I
257
c) Advanced training.
The purpose of an advanced training course is threefold
1) To obtain feedback so that the basic vibration analysis
course can be refined.
2) To confirm all the points that have been taught in the
basic and on-the-job training courses.
3) To upgrade knowledge concerning vibration analysis
techniques and measurements by going into more detail
concerning diagnostics.
d ) Mananerjal awareness
The maintenance superintendents and the power station
managers are row involved in problems which were quite
unknown or did not concern them. It is therefore vitally
important that the abovementioned personnel are aware of
the benefits end advantages of a V.C.M. programme.
Irrespective of the sophistication of the vibration
monitjring equipment and the comprehensiveness of the
training courses a V.C.M. programme will not succeed
unless the maintenance personnel involved are motivated
and dedicated, and top management support the programme.
9.2 CONTINUOUS VERSUS PERIODIC MONITORING
On-line vibration monitoring systems must satisfy certain
requirements i; they are to provide an effective means of
detecting and diagnosing abnormal vibration characteristics
for large turbo-generators, feed pumps and draught group
fans.
25 8
With the advent of computerised real-time systems, vibration
monitoring lends itself to continuous on-1ine-monitoring.
Continuous monitoring requires a relatively large initial
*
capital expenditure, but once installed, cost of operation is
quite low. Periodic muiiitoring on the other hand has a lov
initial cost, but it is manpower intensive and therefore has
a relatively high continuing cost.
9.2.1 Continuous Monitoring
Continuous vibration monitoring is necessary on critical
machines that are subject to problems, or where problems can
develop rapidly and have severe financial consequences.
Continuous monitoring may be dictated by safety
considerations. Even if the cost of a failure is small,
machines should be continuously monitored if a failure will
result in hazards to personnel.
The output of a continuous machinery monitoring system must
be hi *h1y representative of the machine's condition. It must
also e responsive enough to provide warning of impending
problems in time to avoirf maior catastrophic f.iiluren.
Continuous monitoring systems should also allow for the
analysis of transient vibration data, during the run-up and
coastdown of machines so that Bode and Kyquist plots can be
generated. This is suggisted on the main turbo-generator set
for crack detection purposes.
Permanently installed velocity transducers on a FD fan and
the turbo--generator can be seen in Figures 9.1 and 9.2
respect ively.
259
Continuous vibration monitoring involves mounting pf-Tcarent 1y
inrtalled accelerometer*, velocity probe*, proximity probes,
dual probes or shaft rider* to a machine. The information
from these transducers i* relayed continuously to a monitor
or atrip chart recorder or on request in a control room.
Typical *trip chart recorder* can be aeen in Figure 9.3.
9.2.2 Periodic Monitoring
Periodic monitoring is typically applied to less critical
machinery where advance warning of deteriorating conditions
will show a positive return on invetteient. Periodic
monitoring consists of logging measurements at pre-deterrained
intervals using portable hand held meters or portable d£ta
collectors. Typical analysis of the data ranges froir
relatively simple manual logging of overall RMS values to
detailed frequency spectrum analysis of dynamic data from
machine*.
Signals from permanently installed sensors for continuous
monitoring can be used for periodic trending. Most
prrmanently installed sensor*, for various machines, are
connected to a common monitoring panel. This panel consists
of individual modules f-om each of the sensors. These
nodules normally have buffered outputs whereby signals from
the sensors are spectrum analysed .nd compared against
previous baseline signature* u*ing a portable data collector.
A typical control box for permanently installed velocity
transducer* on the draught group fan* can be aeen in
Figure 9.4.
260
A
CONTROL BOa FOR PERMANENTLY INSTALLED VELOCITY TRANSDUCERS ON TIC DRAUGHT GROUP FANS
PERMANENTLY INSTALLED VELOCITY TRANSDUCER ON DC BEARING Of THE DRAUGHT GROUP FANS
Velocity Transducer
FIGURE 9.1
FIGURE 9.3STRIP CHART RECORDERS IN CONTROL ROOM FOR THE VELOCITY TRANSDUCERS
ON THE .IAIN TURBO-GENERAtOR Sr I .
PERMANENTLY INSTALLED VELOCITY TRANSDUCER USED FOR TURBO-GENERAU)R
262
The method of acquiring data used for periodic vibration
monitoring takes on many foims:
a) A semi-skilled labourer armed vith a hard held vibration
meter coupled to an accelerometer or a velocity
transducer, obtains peak, RMS or overall RMS values from
th*> transducer when it is mounted on an established
location and orientation on a bearing casing. The
measurements are usually manually logged. The measuring
points are usually marked on the machine to ensure that
measurements will correlate witV previously obtained
data. Trending of data is also done manually.
b) An operator is provided with a portable tape recorder and
he simply speaks an identification number, location and
the measured value into the microphone. later the tape
is played back and the sprken information transcribed
onto a permanent record.
c) A tape recorder or data collector is taken to a machine
and the vibration spectra from the bearings are recorded
or stored in a bubble memory. The data is then down
loaded to a mass storage device or computer. If a
computer is used, the storing, trending and signature
comparison is alt performed by the software. Thi6 method
can be seen diagramatica11y in Figure 9.5.
Disadvantages of the abovempntioned methods. The letters in
brackets denotes the method which is applicable.
1) Labour intensive (a).
2) The job becomes routine and monotonous. When this occurs
mistakes creep in and interest quickly wanes (a), (b),
(c).
CONFIGURATION OF ROUTE
DATACOLLECTER
PRINTER
PRINTOUT OF ROUTE THROUGH PLANT
DATA COLLECTION
DATA ANALYSIS
FIGURE 9.5
264
3) Efficient trending and analysis of the data becomes
increasingly difficult as the data base expanda. Aa the
vorklc»ad increases the time apent correlating and
trending data decreases so important aspects of a
machines' health ia overlooked (a).
4) No detailed diagnosis or trouble shooting can be
performed by just examining a trend plot of peak or
overall RMS values (a), (b).
Effective monitoring systemc contain a combination of
continuous and periodic monitoring. Although it may sound
abaurd, it is the author's view that a significant portion of
a machine population does not justify any form of condition
monitoring. For example, machines which operate for long
periods of time between failure, and whose failures have
little or no effect on production and can be inexpensively
repaired, do not justify the cost of conditi)n monitoring.
9.2.3 Trending of Vibration Data
Trending of vibration data is one of the most important
aspects of a condition monitoring programme. Historic
vibration measurements are an excellent reference for
severity measurements, because they are specific to the
machine in question. While discrete frequency vibration
levelB for a machine may not be known there u a high
probability that large changes in vibration level indicate a
problem.
The process of monitoring vibration levels or spectrums for
changes is referred to as trend analysis. Since vibration
levels in a good machine are variable, it is not always
PERTiN
ENT
PAR
AM
ETER
265
obvious h.iw much change is tolerable. A typical trend plot
can be * • in Figure 9.6 below.
>END PLOT
TIME
9.2.4 Overall Level Monitoring
FIGURE 9.6
Irrespective of the complexity of the vibration,
overal1-monitoring e x p r , S8es the broad level of vibration at
a measuring point. It is the common first line of approach
to vibtution monitoring and provides basic information for
trend ton Storing.
266
Overall RMS ia defined at the "energy" present in a frequency
spectrum.
Wher* *j ■ some frequency component in a frequency spectiut.
a) This method of treodin" an be used by inexperienced
per srnnel.
>.» The •quipment uaed to obtain the overall levels is
cheap and ccmpact. It normally contist* of a hard
held vibration monitor meter coupled to an
acce 1«. .cmeter or velocity transducer.
c) Outputi rrom permanently installed transducers can
also b* used to obtain overall RMS values.
d) It is or'y effective in detecting elementary defects
such a* unbalance, misalignment and mechanical
looser.* r.
e) Minimal data logging is required. Only one value
per measurug point is required.
K
. i,hemacically overall RMS •
N
ft » total nvmber of value*
.4.1 Adv.inr^ges of Overall Leve! Monitoring:
f) Interpretation* and appraisals of the data can be
based on well established condition acceptability
standards.
Author Ducci PP
Name of thesis Vibration Analysis And Diagnostic Techniques, With Reference To The Implementation Of A Vibration
Condition Monitoring Programme. 1987
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